The present invention relates to a brake system of the type having anti-lock control including a pedal-operated hydraulic braking pressure generator, a braking pressure modulator which is provided in the pressure fluid conduits between the braking pressure generator and the wheel brakes and which serves to vary the braking pressure by changing the volume of a chamber containing the hydraulic medium, sensors for determining the wheel rotational behavior and electronic curcuits for processing the sensor signals and for generating braking-pressure control signals. The invention also relates to brake systems which include both anti-lock control and traction slip control and to such brake system including braking pressure modulators for controlled vehicular braking.
Brake systems of this type which operate according to a principle known as the plunger principle are known. In such systems, as soon as the wheel rotational behavior displays a tendency to lock, the pressure fluid conduit from the braking pressure generator to the wheel brake affected will be closed and the volume of a chamber connected with the affected wheel brake and containing the enclosed hydraulic medium will be expanded by the action of an auxiliary force. For this purpose it is conventional practice to use a piston or plunger piston whose end surface confines a chamber hydraulically communicating with the wheel brake, and which is displaceable by the braking-pressure control signals with the aid of an extraneous force or auxiliary force. To re-increase the braking pressure during a control action, the piston is reset into its original position.
Such a brake system is described for instance in the German published patent application No. P 34 37 994. The braking pressure generator of that system includes a master brake cylinder to which the wheel brake of the controlled wheel is hydraulically connected. Inserted into the pressure fluid conduit for achieving pressure modulation is an adjusting element comprising a plunger piston and a spherical seat valve which valve, in the adjusting element's inactive position, is kept open by a tappet fastened to the end surface of the piston. A strong spring maintains the piston in its initial position in which position the adjusting element does not influence the braking operation. To displace the piston in opposition to the force of this spring, so as to achieve the desired pressure decrease during a control action, the piston is connected with an electric motor by way of a male screw and an eddy-current clutch. The clutch is essentially composed of a cogwheel driven by the electric motor, a stationary induction coil and a rotor which is connected to the male screw and with the plunger piston. The torque exerted on the rotor, which causes retraction of the plunger piston and thereby results in the volume increase and corresponding reduction in braking pressure, is dependent on the magnitude of the induction current in the induction coil which, when the cogwheel is rotating, generates a rotating field. Resetting of the piston is effected by the resetting spring. Variation of the induction current which flows through the induction coil provides for control of the braking pressure variation.
As soon as the plunger piston is displaced in opposition to the force of the resetting spring as a result of the torque generated, the spherical seat valve will close and thereby interrupt the pressure fluid conduit leading from the braking pressure generator to the pressure modulator and to the wheel brake.
Further, the anti-lock hydraulic brake system disclosed in German published patent application No. P 36 02 430 has each brake circuit furnished with a plunger system having a stepped piston that is applied by the braking pressure and which is kept in its initial position by a resetting spring in the event of braking operations without slip control. The auxiliary energy for displacing the plunger piston, which must overcome the force of the resetting spring, is supplied by a hydraulic pump which includes a pressure fluid conduit for each plunger system. One shortcoming of such brake systems is that two or more plunger systems are required which increases complexity of the system.
Another known brake system is disclosed in the German published patent application No. P 33 17 629 which includes a master cylinder and a vacuum or hydraulic booster inserted upstream of the master cylinder for boosting the pedal force and is designed such that, in a slip control action, the auxiliary-force-assisted pedal force directed to the master cylinder is temporarily partially or fully compensated by an opposing extraneous force. The controlled wheels are connected to the master cylinder by way of multidirectional control valves which are normally open and which can be switched over to close. According to the so-called Multiplex method, the individual wheels are consecutively connected to the master cylinder by means of these multidirectional valves. By corresponding adjustment and variation of the opposing extraneous force, the braking pressure in the wheel which is at any time in hydraulic communication with the master cylinder can be decreased or increased to the pressure level desired. The generation and quick variation of the extraneous force which counteracts the pedal force in such systems necessitates an elaborate braking pressure generator design. Further these known brake systems are not adapted for traction slip control.